1. Field
The present disclosure relates to integrated plasmon detectors.
2. Related Art
When a species of atoms (whether gas, liquid, or solid) is ionized into an equal number of free electrons and ionized atomic cores known as ions, the atoms are said to be in a plasma state. In an ideal undisturbed gaseous plasma, the density of free electrons is equal to that of the positively charged ions and the overall distribution of charge is equal, and thus neutral, throughout the plasma. When this distribution is disturbed, the electrons seek to restore their neutral positions through the combined effect of repulsion from other electrons and attraction from the uniform positive charge background of the ions. This will induce an oscillation in the electrons as they attempt to return to their neutral positions known as plasma oscillation. [The Feynman Lectures of Physics, R. P. Feynman et al., Addison Wesley, Reading, Mass. 1964, the entire contents of which are incorporated herein by reference.]
In a metal, the density of free electrons is much higher, and their temperature much lower, than in a gaseous plasma. These free electrons are thus a quantum gas and, when oscillating, form what is termed a plasmon. Free electrons oscillating at a common frequency are oscillating at plasmon frequencies that are generally very high, having a typical value on the order of 3×1015 Hz (corresponding to a charge of about 12 eV). [Elementary Excitation in Solids, D. Pines, Benjamin, N.Y. 1964; and Statistical Mechanics, R. P. Feynman, Addison Wesley, Reading, Mass. 1972; the entire contents of both of which are incorporated herein by reference.] For purposes of discussion and with reference to FIG. 1(a), whereas a plasmon is understood herein to refer to the state of quantum plasma in a solid, a jellium 2 is understood to mean a quasiparticle consisting of a negatively charged core 4 shielded by positive charges 6 gathered from the surrounding ions within a Fermi-Thomas radius λFT, which is comparable to the radius of a host atom in a metal lattice. The electron thus oscillates within this atom-sized sphere of positively-charge volume, evincing a high frequency and thus displacement that is small relative to the size of the sphere. When all such electrons oscillate in phase with one another, a standing plasmon wave arises (k=0), whereas a linear series of electrons having a definite phase relationship to one another correspond to a traveling plasmon wave having definite direction and mode numbers k (k≠0).
A quantum plasma in a solid also contains individual “hot” electrons that tend to interact (i.e. collide) with each other and with jelliums much more frequently that with the host ion lattice. When a hot electron 4′ penetrates a jellium 2, as shown in FIG. 1(b), there are two negative electron charges 4, 4′ inside a volume 6 of a unit of positive charge. This imbalance of charge leads the jellium to disintegrate by the expulsion of both electrons such that total momentum is conserved. Conversely, when two such hot electrons collide, as shown in FIG. 1(c), the result is a stationary jellium 2 at the point of impact and one free electron 4′. Molecular physics teaches us that the probabilities of these two opposite processes are equal.
When a metal is impinged upon by a laser pulse beam having a frequency below the plasmon frequency of the metal, electrons begin to be set in motion at randomly distributed frequencies lying between the laser beam and the plasmon frequencies (between 1015 and 3×1015 Hz). Initially most of these electrons are free hot electrons, with few jelliums. These hot electrons tend to favor the creation of jelliums through their collisions, and thus the subgroup of collective electron plasmonic oscillations begins to build up in jelliums as energy is transferred from the laser beam to the plasmon system. Depending on the length of the laser pulse and the thickness of the metal, the plasmon oscillations may reach a peak maximum range, with free electron density as high as 1023/cm3. These collective oscillations have a natural frequency or plasma frequency determined by the density of electrons in the neutral distribution n0, and can be expressed as
                    f        =                              (                          1                              2                ⁢                π                                      )                    ⁢                                                                      ⅇ                  2                                ⁢                                  n                  0                                                                              ɛ                  0                                ⁢                                  m                  e                                                                                        (        1        )            where e is the unit electron charge, n0 is the neutral density of electrons in a plasma, ε0 is the permittivity of vacuum, and me is the unit electron mass.
When the laser beam ceases to impinge onto the metal, most jelliums continue to oscillate at their respective plasmon frequency characteristic of concentration and movement (mood number). When a jellium falls out of step with the whole class collective modes of existing plasmon oscillations, it drops out and an ‘individual’ hot electron (as opposite to a ‘collective class’ hot electron) results that eventually cools down to room temperature to become a thermal electron. However, if it does not pass through an adaptor layer to cool down quickly, the remaining heat will make detecting it functionally difficult.
Currently known methods and devices for measuring the decay of a plasmon all rely on photodetectors of various types to detect the emitted decay photons. This approach is limited by the fact that the decay photons have to travel a relatively long path from the surface to the detector, a path over which they undergo angular spreading misalignment, and environment influences. Thus, collection and detection of decay photons as a means of studying plasmon effects can be difficult and prone to inaccuracies. The present disclosure addresses these difficulties with a novel approach to plasmon detection: monitoring the hot electrons internally created by plasmon decay.